Cu2ZnSnS4 Thin Film Solar Cells Research Articles

Numerical Optimization of Thickness and Optical Band Gap of Absorber and Buffer Layers in Earth-Abundant Cu2ZnSnS4 Thin-Film Solar Cells

Numerical Optimization of Thickness and Optical Band Gap of Absorber and Buffer Layers in Earth-Abundant Cu2ZnSnS4 Thin-Film Solar Cells

Exploring graphene's potential as a transparent conductive layer in Cu2ZnSnS4 superstrate solar cells

The fabrication of environmentally friendly, semi-transparent, high-performance and cost-effective inorganic solar cells has been the subject of recent extensive study. One area of study involves incorporating one-dimensional nanostructures and high quality transparent conductive layers into the conventional thin film solar cell systems. The objective of this particular investigation was, therefore, to construct such a structure by integrating Graphene-ZnO-Nanorods (NRs) hybrid structure into a conventional Cu2ZnSnS4 (CZTS) thin film solar cell architecture. The process involved synthesizing vertically-aligned ZnO NRs, coated with thin layers of SnO2 and CdS, on chemical vapor deposited graphene pre-coated glass substrates. Following the SnO2-passivation and CdS coating, vertically well-aligned ZnO NRs were then decorated with a 500 nm-thick layer of CZTS using a one-step thermal evaporation technique.This process led to the manufacture of a superstrate solar cell with SLG /Graphene/ZnO-NRs/CdS/CZTS/Ag device structure as an example of graphene’s application in optoelectronic devices. To reveal the physical properties of the grown graphene and deposited CZTS thin films, they were subjected to various characterization techniques. The structural, chemical and optical analyses results showed the formation of a single-phase kesterite CZTS thin film with a copper-deficient composition and an optical band gap of 1.47 eV on glass substrate and single layer growth of graphene on Cu-foil substrate, which was subsequently successfully transferred onto glass substrates. Electrical measurements unveiled the existence of two different VCu point defects in CZTS with thermal activation energies of 45 meV and 180 meV. The manufactured superstrate solar cell exhibited a short-circuit current density of 9.34 mA/cm2, an open-circuit voltage of 390.6 mV, a fill factor of 17.2%, and an energy conversion efficiency of 0.63%.

Effects of pre-alloying process on the absorber quality for Cu2ZnSnS4 thin-film solar cells

Effects of pre-alloying process on the absorber quality for Cu2ZnSnS4 thin-film solar cells

Growth and characterization of spray-deposited Cu2ZnSnS4 thin films by using two different carrier gases

Cu2ZnSnS4 is a promising renewable energy thin film semiconducting material used in heterojunction solar cells. These films have been deposited at the optimized condition of the spray pyrolytic technique. Compressed air and nitrogen are used as carrier gases for the deposition of the films. The X-ray diffraction studies and micro-Raman analysis confirm the structure of these films as kesterite. The crystallinity of the films depends on the nature of the carrier gas. The microstrain and dislocation density in the films is found to decline when the films deposited with nitrogen are used as carrier gas. The optical studies confirm these materials exhibit a direct band gap and are close to 1.53 eV. The material shows p-type conductivity. A heterojunction Cu2ZnSnS4 thin film solar cell is fabricated using simple chemical methods and this cell shows a maximum efficiency of about 0.8%.

12.3% Efficient Low Voc Loss Pure Sulfide Kesterite Solar Cells from DMSO Solution via Cadmium Alloying

AbstractCd alloying has been theoretically proved to be an effective strategy to suppress Cu‐Zn antisite defects and related defect cluster for improving device performance of pure sulfide kesterite Cu2ZnSnS4 (CZTS) thin film solar cells. However, the potential of Cd alloying has not been fully realized by solely doping without further post heat‐treatment. Here, Cd alloying CZTS (Cu2(Zn,Cd)SnS4, CZCTS) is reported through dimethyl sulfoxide (DMSO) solution and how alloying concentration affects reaction path, grain growth, and electronic properties of the CZCTS absorbers is investigated. This study found that Cd can be incorporated into CZTS through direct phase transformation grain growth, which sufficiently suppresses band tailing. High quality CZCTS absorber films and efficient solar cells are fabricated within a wide range of alloy concentration. A champion CZCTS device with a power conversion efficiency of 12.3% is achieved at 35% Cd concentration without any post heat treatment, improved by over 70% compared to 7.0% of CZTS. This device exhibits a high VOC gain to the Shockley–Queisser (Voc/VocSQ = 59.7%), the lowest VOC deficit achieved in pure sulfide kesterite solar cells. The results demonstrate the importance of the Cd alloying strategy for mitigating band tailing and achieving high efficiency pure sulfide kesterite solar cells.

The effect of step cooling treatment on the microstructure of Cu2ZnSnS4 film and its electric property

Flexible Cu2ZnSnS4 (CZTS) thin-film solar cells have the advantages of high flexibility, lightweight, and easy storage, which can provide portable and clean energy for various portable electronic and communication devices and vehicles. In this work, CZTS thin film deposited on flexible aluminum foil were prepared by vacuum magnetron sputtering and sulfide annealing in a tubular furnace. A step-by-step cooling way was applied to the annealing stage aiming at tuning the formation of CZTS, that is, when the slow cooling reached a target temperature (400 °C, 350 °C, 300 °C, 250 °C), a rapid cooling was triggered by opening the tube furnace lid. The effects of step cooling on the micro structure and property of CZTS film were studied by X-ray diffraction, Raman scattering, energy dispersive spectroscopy, scanning electron microscopy and current-voltage (I-V) characteristic curve test. The experimental results showed that all samples have CZTS phase and the Al/CZTS interface exhibited ohmic contact. And the formation of secondary phases like SnS, Sn2S3 and Cu31S16 in the sample film can be tuned by the step cooling treatments. The sample treated at 300 °C had the highest CZTS content, the average grain size of CZTS was 45 nm and the number of secondary phases was fewer. Beside, the I-V curve revealed that this sample had the lowest resistance. The experimental results in this work indicate the feasibility of Al as a substrate and back electrode for CZTS thin film.

Improved performance of Cd-free CZTS thin-film solar cells by using CZTS0.4Se0.6 BSF layer

Cu2ZnSnS4 (CZTS) thin film solar cells (TFSCs) have received great attention from the solar cell industry for their environment, price, high absorption coefficient and great electronic properties. This work provides a strategy to prompt the photoelectric conversion efficiency (η) of CdS/CZTS-based TFSCs via introducing the back surface field (BSF) layer and wxAMPS to simulate the results. Meanwhile, the optimum ratio of sulfur to selenium in the BSF layer material CZTSSe has been also studied, and a new device structure has been constructed. Beneficial from the introduction of CZTSSe as a BSF layer, the interface recombination is suppressed and leads to an enhanced Voc. Subsequently, MoO3, ZnS, WO3, TiO2 and In2S3 as candidates to substitute for the toxic Cd buffer layer are investigated. The results demonstrate that In2S3/CZTSSe exhibits the best performance with 28.59 % and 0.99 V, ascribed to a superior spike-like value. It was found that the presence of a spike-like conduction band (CB) at the In2S3/CZTSSe interface and a more suitable conduction band offset (CBO) value could suppress the interfacial complexation. Therefore, In2S3 performs best as a buffer layer material. However, due to the cost of indium, this paper recommends the utilization of wide bandgap semiconductor materials MoO3 and ZnS, which also have good stability and conversion efficiencies that can reach 28.38% and 28.41%, respectively. The work of this paper provides important guidance for researchers to manufacture CZTS TFSCs.

CuSCN Modified Back Contacts for High Performance CZTSSe Solar Cells

AbstractOptimization of the back contact interface is crucial for improving the performance of Cu2ZnSnS4 (CZTS) thin film solar cells. In this paper, self‐depleted CuSCN is deployed as an intermediate layer at the Mo/CZTS interface to improve the quality of the back contact. This CuSCN layer, obtained via aqueous solution processing, reduces the thickness of Mo(S,Se)2 and eliminates multi‐layer crystallization of the absorber by suppressing the undesirable reaction between Mo and Se during the selenization process. By regulating the selenium infiltration into the CZTS precursor films during the selenization process, highly crystalline, single‐layer Cu2ZnSn(S,Se)4 (CZTSSe) absorber layers are realized. The single‐layer CZTSSe absorber exhibits reduced carrier recombination, enhanced carrier density and increased work function. The improved back contact and absorber layer enables 11.1% power‐conversion‐efficiency to be achieved.

Atomic Layer Grown Zinc–Tin Oxide as an Alternative Buffer Layer for Cu2ZnSnS4-Based Thin Film Solar Cells: Influence of Absorber Surface Treatment on Buffer Layer Growth

Zn1-x Sn x O y (ZTO) deposited by atomic layer deposition has shown promising results as a buffer layer material for kesterite Cu2ZnSnS4 (CZTS) thin film solar cells. Increased performance was observed when a ZTO buffer layer was used as compared to the traditional CdS buffer, and the performance was further increased after an air annealing treatment of the absorber. In this work, we study how CZTS absorber surface treatments may influence the chemical and electronic properties at the ZTO/CZTS interface and the reactions that may occur at the absorber surface prior to atomic layer deposition of the buffer layer. For this, we have used a combination of microscopy and synchrotron-based spectroscopies with variable information depths (X-ray photoelectron spectroscopy, high-energy X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy), allowing for an in-depth analysis of the CZTS near-surface regions and bulk material properties. No significant ZTO buffer thickness variation is observed for the differently treated CZTS absorbers, and no differences are observed when comparing the bulk properties of the samples. However, the formation of SnO x and compositional changes observed toward the CZTS surface upon an air annealing treatment may be linked to the modified buffer layer growth. Further, the results indicate that the initial N2 annealing step integrated in the buffer layer growth by atomic layer deposition, which removes Na-CO x species from the CZTS surface, may be useful for the ZTO/CZTS device performance.

Cu2ZnSnS4 thin-film solar cells by a closed tube sulfurization under saturated sulfur vapor pressure

CdS/Cu2ZnSnS4 (CZTS) heterojunction thin-film solar cells were fabricated via a closed tube sulfurization under saturated sulfur vapor pressure. A significant improvement in the crystalline quality of the CZTS polycrystalline thin-film was achieved when the Cu–Zn–Sn metallic-alloy thin-film was sulfurized under saturated sulfur vapor pressure of 4.3 atm at 590 °C due to efficient suppression of re-evaporation of Zn and Sn species during the growth process. Accordingly, the open-circuit voltage and short-circuit current density of the CdS/CZTS heterojunction device were enhanced resulting in the improvement of power conversion efficiency by over 1.7 times compared to the best cell fabricated by the conventional processes under unsaturated sulfur vapor pressure of 1.5 atm.

Growth of Cu2ZnSnS4 Thin Film Solar Cells Using Chemical Synthesis

Objectives: Cu2ZnSnS4 thin film solar cell is fabricated by using the chemical spray pyrolysis method. Method: CZTS thin films were successfully deposited by employing the eco-friendly and cost-effective chemical spray pyrolysis method. X-ray diffraction (XRD) and Raman spectroscopy are used to investigate the structural properties of the prepared samples. The surface morphology of the deposited films is studied using a scanning electron microscope. An energy dispersive spectrometer is used to know the chemical compositions of the deposited films. A double beam spectrophotometer is used to study the optical characteristics of these films. The hot probe technique is used to confirm the nature of the conductivity of the deposited films. Findings: The XRD spectra of the thin films reveal the polycrystalline nature with the kesterite structure of the thin films. The optical band gap is found to be 1.52 eV and the absorption coefficient value of these films is found to be  104 cm-1. The nature of conductivity is identified to be p-type. Finally, Cu2ZnSnS4 thin film solar cell is fabricated with the substrate configuration glass/Mo/CZTS/CdS/Ag which exhibited an open circuit voltage and short circuit current of 156 mV and 1.82 mA/cm2 respectively. Novelty: The growth of Cu2ZnSnS4 thin films is studied by varying the distance between the spray nozzle and glass substrate from 25 to 35 cm in steps of 5 cm and maintaining all other parameters as constant. Finally, the Cu2ZnSnS4 thin film solar cell is prepared in a substrate as well as a superstrate configuration. Keywords: Thin films; Absorber layers; Spray technique; Solar cell; Cu2ZnSnS4

Inhibiting the formation of MoS2 between Cu2ZnSnS4 thin film and Mo(211) foil substrate by inserting a Mo(110) intermediate layer

In Cu2ZnSnS4 (CZTS) thin film solar cells, a MoS2 layer is often found at the CZTS/Mo interface, which limits the photovoltaic performances of solar cells. In this work, a Mo (110) intermediate layer was inserted between the CZTS thin film and Mo (221) foil substrate to inhibit the formation of MoS2 layer. The experimental results showed that a MoS2 layer with a thickness of 2.0 μm was observed at the CZTS/Mo foil interface without Mo intermediate layer. After the insertion of a sputtered Mo (110) intermediate layer between CZTS and Mo foil substrate, the thickness of MoS2 reduced as the thickness of Mo intermediate layer increased. The MoS2 interface layer can not be detected from the cross-sectional SEM images when the thickness of Mo intermediate layer exceeded 277 nm. The inhibition of MoS2 was considered to be due to the chemical inertness of Mo (110) during sulfurization. The ohmic contact characteristic of CZTS/Mo foil was improved when the Mo layer had a suitable thickness. Therefore, the interface properties between CZTS and Mo (221) foil can be enhanced by using a Mo (110) intermediate layer.

Photovoltaic performance of Cu2ZnSnS4 thin film solar cells on flexible molybdenum foil formed by electrodeposition and sulfurization

Photovoltaic performance of Cu2ZnSnS4 thin film solar cells on flexible molybdenum foil formed by electrodeposition and sulfurization

Band alignment tuning at Mo/CZTS back contact interface through surface oxidation states control of Mo substrate

The molybdenum (Mo) back contact is one of the crucial factor that affects the performance of kesterite Cu2ZnSnS4 (CZTS) thin film solar cells. Thick MoS2 which is formed during sulfurization process of CZTS is known to be harmful for the device performance. MoOx on surface of Mo substrate with proper thickness can enhance the efficiency of CZTS solar cells through reducing the thickness of MoS2 layer and tailoring the work function of back contact. However, the role of oxidation states of Mo in MoOx on the performance of solar cells is unclear. Herein the effect of MoOx with different oxidation states on CZTS absorber and intermediate MoS2 layer were investigated. Our results shown that the efficiency of CZTS solar cells on Mo substrate with proper surface oxidation status (MoO2.03) can be as high as 2.86 times of the one fabricated on higher oxidized Mo (MoO2.21), although the thinnest MoS2 layer was present in the latter case. This work discloses the origin of the oxidation states of Mo for the efficiency enhancement of kesterite structured solar cells. The results will be also helpful to improve the performance of compound semiconductor by oxidation states control strategy.

Solution-processed pure Cu2ZnSnS4/CdS thin film solar cell with 7.5% efficiency

Abstract The kesterite Cu2ZnSnS4 (CZTS) thin-film solar cell is one of the emerging solar cells. The Zn and Sn materials were prepared to substitute the rare In and Ga materials in previous CuInGaS2 cell generation. However, the secondary phase formation and the non-uniform film formation during the solution process affect deteriorating cell performance. By considering the CZTS kesterite structure, secondary phases, bandgap, and absorbance, this work aims to present the novel achievement of solution processed-pure Cu2ZnSnS4/CdS solar cell. The solar cell was made with the structure of Mo/Cu2ZnSnS4/CdS/ITO/Ag consisting of 21 subcells. The characterizations were employed through X-ray diffraction (XRD), external quantum efficiency, energy gap, and J-V curve. To the best knowledge, without further cation exchange modification and the atomic ratio of Zn/Sn = 1.26 and Cu/(Zn + Sn) = 0.86, the fabricated solar cell exhibits the novel achievement for the pure Cu2ZnSnS4/CdS standard solution process with the power conversion efficiency of 7.5% with 1.43 eV bandgap and 83.5% external quantum efficiency at 570 nm. The correlation between kesterite, secondary phases, bandgap, absorbance, dan cell performance statistics were further discussed. This work implied considering the atomic ratio of Cu-poor, Zn-rich, and the film uniformity during the spin coating process; it plays a significant role in achieving high-performance solution-processed CZTS solar cells.

Modification of Back Contact in Cu2ZnSnS4 Solar Cell by Inserting Al-Doped ZnO Intermediate Layer.

The optimization of back contact interface is crucial to improve the performance of Cu2ZnSnS4 (CZTS) thin film solar cells. In this paper, we first employ Al-doped ZnO (AZO) as the intermediate layer into the Mo/CZTS interface to improve the quality of back contact region. This AZO intermediate layer, obtained from the sputtering method prior to the CZTS precursor deposition, initially blocks the direct contact of CZTS with the Mo layer and thus indeed suppresses the decomposition reaction between Mo and CZTS. Consequently, the generation of voids at the back contact region is obviously avoided. Besides, the AZO intermediate layer can inhibit the reaction between sulfur (S) and Mo during sulfurization process, and thus significantly reduce the thickness of MoS2. Meanwhile, the AZO intermediate layer with suitable thickness does not affect the crystal quality of CZTS absorber layer. Moreover, the effects of different thicknesses of predeposited AZO on the film morphology, composition, and corresponding device performance were systematically studied. After optimizing the thickness of the AZO layer, the efficiency of the resultant device has increased from 7.1% to 8.4% (the active area efficiency is 9.2%).

Articulating effect of low copper content on structure and optoelectronic properties of spray deposited Cu2ZnSnS4 thin films – From experiment and first-principles investigations

Improvement in the efficiency of truly inexpensive spray-pyrolysed Cu2ZnSnS4 (CZTS) thin film solar cells demands both the minimal presence of secondary phases as well as low carrier concentration. The present work deals with a strategy to obtain high quality spray pyrolysed CZTS films with low carrier concentration by controlling Cu-precursor concentration ([Cu2+]). By reducing [Cu2+] from 0.0136 M to 0.0129 M, film’s microstrain changes from compressive (1.76 × 10−3) to tensile (5.89 × 10−3) in nature, allowing the suppression of secondary phase formation. At an optimally low [Cu2+] of 0.0122 M the film shows pure phase, a lower tensile strain and a free carrier concentration of 3 × 1018 cm−3 with p-type conductivity and a high mobility of 3.22 cm2/Vs. Interfacial charge transfer is also promoted in this condition which is verified from electrochemical impedance spectroscopy. Moreover, the consequence of the modulation in the electrical properties through precursor [Cu2+] on the electronic structure and its optical absorption properties are corroborated from first-principles calculation.

CZTS thin film solar cells on flexible Molybdenum foil by electrodeposition-annealing route

Earth-abundant and non-toxic Kesterite-based Cu2ZnSnS4 (CZTS) thin film solar cells are successfully fabricated on flexible Molybdenum (Mo) foil substrates by an electrodeposition-annealing route. A well-adherent, densely packed, homogeneous, compact, and mirror-like CZT precursor is initially produced through electrodeposition by using a rotating working electrode. Subsequently, the co-electrodeposited CuZnSn (CZT) precursor is sulfurized in quartz tube furnace at 550 °C for 2 h in N2 atmosphere with the presence of elemental sulfur in order to form CZTS. Different characterization techniques like XRD, SEM, HR-TEM, Raman, and Photoluminescence demonstrate that almost phase-pure CZTS formed after sulfurization. A flexible Al/Al-ZnO/i-ZnO/CdS/CZTS/Mo foil solar cell is produced, where CdS is deposited by chemical bath deposition and transparent conducting oxide (TCO) is deposited by DC sputtering. The CZTS solar device shows a 0.55% power conversion efficiency on flexible Mo foil substrate and it constitutes the first prototype of this kind of solar cell produced by electrodeposition-annealing route without any surface modification of the Mo substrate.Graphic abstract

EFFECT OF TIN PRECURSORS ON THE DEPOSITION OF Cu2ZnSnS4 THIN FILMS

Cu2ZnSnS4 is a potential compound semiconducting material for solar absorber layer in thin film heterojunction solar cells. Chemical spray pyrolysis technique has been successfully employed to deposit these thin films using two different tin precursors. Thin films were characterized by studying their structural, composition, electrical and optical properties. X-ray diffraction pattern reveals that these films exhibit polycrystalline nature with kesterite structure. Lattice parameters of Cu2ZnSnS4 films are found to be a = b = 0.544 nm and c = 1.084 nm. Optical band gap, evaluated from spectral transmittance data, is close to ideal energy gap (1.5 eV) exhibit highest conversion efficiency. Optical absorption coefficient of these films is ≥ 104 cm-1 . These films exhibit p-type nature. A humble attempt is made to fabricate a typical heterojunction Cu2ZnSnS4 thin film solar cell.

The preparation of Cu2ZnSnS4 thin film solar cell based on oxygen containing precursor

In this paper, we reported a new method to fabricate Cu2ZnSnS4 (CZTS) thin film solar cells. Oxygen containing precursor thin films were deposited on the Mo-coated soda lime glass (SLG) by sputtering ZnO, SnO2 and Cu targets. For getting higher quality CZTS thin films, the effects of different annealing temperatures of 380 °C, 480 °C, 580 °C and 650 °C on CZTS films were systematically investigated. The results showed that the optimum annealing temperature was 580 °C. Through optimizing the sulfurization process, phase-pure CZTS thin films with compact and large grains were obtained. Composition analysis showed that O was not detected after sulfurizing process which indicated all of O was replaced by S. And due to the stability of SnO2, the loss of tin can be avoided. In addition, ZnO not only can provide zinc sources, but also can decrease the thickness of MoS2 as a barrier layer. Finally, CZTS solar cells based on sputtering oxides targets were obtained and the preliminary fabricated CZTS thin film solar cells had an efficiency of 5.08%, with an open-circuit voltage of 645 mV, a short-circuit current density of 21.74 mA/cm2 and a fill factor of 36.23%.